There are several reasons that ink-jet printing has become a popular way of recording images on various media surfaces, particularly paper. Some of these reasons include low printer noise, capability of high-speed recording, and multi-color recording. Additionally, these advantages can be obtained at a relatively low price to consumers. Though there has been great improvement in ink-jet printing, accompanying this improvement are increased demands by consumers in this area, e.g., higher speeds, higher resolution, full color image formation, increased stability, improved durability, etc.
As new ink-jet inks are developed, there have been several traditional characteristics to consider when evaluating the ink in conjunction with a printing surface or substrate. Such characteristics include edge acuity and optical density of the image on the surface, black-to-color bleed control, dry time of the ink on the substrate, adhesion to the substrate, lack of deviation in ink droplet placement, presence of all dots, waterfastness, long term storage stability, and long term reliability without corrosion or nozzle clogging, and long term print durability. Though the above list of characteristics provides a worthy goal to achieve, there are difficulties associated with satisfying all of the above characteristics. Often, the inclusion of an ink component meant to satisfy one of the above characteristics can prevent another characteristic from being met. Thus, most commercial inks for use in ink-jet printers represent a compromise in an attempt to achieve at least an adequate response in meeting all of the above listed requirements.
For example, ink-jet inks containing pigments generally require a relatively high pH in order to keep the pigments dispersed within the ink-jet ink. Acrylic acid copolymers are commonly used to disperse pigments in aqueous ink-jet inks. These copolymers typically have carboxylic acids which help to stabilize the pigments, but which also have a high pKa. Therefore, depending on the specific formulation, as the pH drops below about 9, electrostatic stabilization provided by the carboxylate groups decreases. As a result, the pigments are no longer properly dispersed and tend to flocculate and settle. Further, ink-jet inks with high pH can cause excessive corrosion or degradation of various parts of the ink-jet pen architecture. In addition, upon long term storage of initially stable dispersions, carbon dioxide can be absorbed into the ink-jet ink composition causing the pH to drift downward, resulting in a gradual loss of a stable pigment dispersion.
Methods have been sought to avoid these problems with varying success. For example, pigments have been dispersed using cationic polymers or a variety of steric stabilization components to prevent pigments from coagulating. Such methods often result in very low, i.e. acidic, pH in which the ink-jet pen is also susceptible to corrosion. As such, investigations continue in improving these characteristics, while also maintaining good ink-jet architecture reliability.